Effects of nitrate on phosphorus release from lake sediments

Phosphorus (P) release from sediment is a key process affecting the effectiveness of eutrophication mitigation. We hypothesized that high nitrate (NO₃^-) input may have dual effect on sediment P release: reduce the sediment P release by improving the oxidation of sediment or promote P release by stimulating the growth of phytoplankton and increase the decomposition rates and oxygen consumption at the sediment water interface. To test the effect of different NO₃^- concentrations, we conducted a three-month experiment in 15 cement tanks (1 m³), with five targeted concentrations of NO₃^-: control, 2 mg L^-1, 5 mg L^-1, 10 mg L^-1, and 15 mg L^-1. The results showed that: i) when NO₃^- was maintained at high levels: NO₃^-≥5-7 mg L^-1 (range of median values), there was no effect of NO₃^- on net P release from the sediment, likely because the positive effects of NO₃^- (increasing oxidation) was counteracted by a promotion of phytoplankton growth. ii) after NO₃^- addition was terminated NO₃^- dropped sharply to a low level (NO₃^-≤0.4 mg L^-1), followed by a minor P release in the low N treatments but a significant P release in the high N treatments, which likely reflect that the inhibition effect of NO₃^- on P release decreased, while the promotion effects at high NO₃^- concentrations continued. The results thus supported our hypotheses of a dual effect on sediment P release and suggest dose-dependent effect of NO₃^- loading on stimulating P release from the sediment, being clear at high NO₃^- exceeding 5-7 mg L^-1.